The present invention relates to novel mesoporous, lamellar silica compositions and to a method for the preparation thereof. In particular the present invention relates to the use of novel gemini amine surfactants as templating or structure directing agents.
Porous materials created by nature or by synthetic design have found great utility in all aspects of human activity. The pore structure of the solids is usually formed in the stages of crystallization or subsequent treatment. Depending on their predominant pore size, the solid materials are classified as: (i) microporous, having pore sizes  less than 20 xc3x85; (ii) macroporous, with pore sizes exceeding 500 xc3x85; and (iii) mesoporous, with intermediate pore sizes between 20 and 500 xc3x85. The use of macroporous solids as adsorbents and catalysts is relatively limited due to their low surface area and large non-uniform pores. Microporous and mesoporous solids, however are widely used in adsorption, separation technology and catalysis.
Owing to the need for higher accessible surface area and pore volume for efficient chemical processes, there is a growing demand for new, highly stable mesoporous materials. Porous materials can be structurally amorphous, paracrystalline, or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess long range order, whereas paracrystalline solids, such as xcex3- or xcex7-Al2O3 are quasiordered as evidenced by the broad peaks on the x-ray diffraction patterns. Both classes of materials exhibit a broad distribution of pores predominantly in the mesoporous range. This wide pore size distribution limits the shape selectivity and the effectiveness of the adsorbents, ion-exchanges and catalysts prepared from amorphous and paracrystalline solids.
Hereafter, in order to clarify one of the objects of the present invention, the terms framework-confined uniform porosity and textural porosity are defined and differentiated. Framework-confined uniform pores are pores formed by nucleation and crystallization of the framework elementary particles. These pores typically are cavities and channels confined by the solid framework. The size of the cavities and channels, i.e. the size of the framework-confined uniform pores, in molecular sieve materials is highly regular and predetermined by the thermodynamically favored assembly routes. The framework-confined pores of freshly crystallized product are usually occupied by the template cations and water molecules. While water molecules are easily removed by heating and evacuation the ionic templating materials, such as quaternary ammonium cations, due to their high charge density, are strongly bonded or confined to the pore cavities and channels of the negatively charged framework. The same concepts are expected to apply for the charge reversed situation where an anionic template is confined in the pores of a positively-charged framework. Therefore, a cation or anion donor or ion pairs are necessary in order to remove the charged template from the framework of the prior art molecular sieves.
Textural porosity is the porosity that can be attributed to voids and channels between elementary particles or aggregates of such particles (grains). Each of these elementary particles in the case of molecular sieves is composed of certain number of framework unit cells or framework-confined uniform pores. The textural porosity is usually formed in the stages of crystal growth and segregation or subsequent thermal treatment or by acid leaching. The size of the textural pores is determined by the size, shape and the number of interfacial contacts of these particles or aggregates. Thus, the size of the textural pores is usually at least one or two orders of magnitude larger than that of the framework-confined pores. For example, the smaller the particle size, the larger the number of particle contacts, the smaller the textural pore size and vice versa. One skilled in the art of transmission electron spectroscopy (TEM) could determine the existence of framework-confined micropores from High Resolution TEM (HRTEM) images or that of framework-confined mesopores from TEM images obtained by observing microtomed thin sections of the material as taught in U.S. Pat. No. 5,102,643.
One skilled in the art of adsorption could easily distinguish and evaluate framework-confined uniform micropores by their specific adsorption behavior. Such materials usually give a Langmuir type (Type I) adsorption isotherm without a hysteresis loop (Sing et al., Pure Appl. Chem. vol. 57, 603-619 (1985)). The existence of textural mesoporosity can easily be determined by one skilled in the art of SEM, TEM and adsorption. The particle shape and size can readily be established by SEM and TEM and preliminary information concerning textural porosity can also be derived. The most convenient way to detect and assess textural mesoporosity is to analyze the N2 or Ar adsorption-desorption isotherm of the solid material. Thus, the existence of textural mesoporosity is usually evidenced by the presence of a Type IV adsorption-desorption isotherm exhibiting well defined hysteresis loop in the region of relative pressures Pi/Po  greater than 0.4 (Sing et al., Pure Appl. Chem. 57 603-619 (1985)). This type of adsorption behavior is quite common for a large variety of paracrystalline materials and pillared layered solids.
The only class of porous materials possessing rigorously uniform pore sizes is that of zeolites and related molecular sieves. Zeolites are microporous highly crystalline aluminosilicates. Their lattice is composed by TO4 tetrahedra (T=Al and Si) linked by sharing the apical oxygen atoms. Oriented TO4 tetrahedra, consists of cavities and connecting windows of uniform size (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London (1974)). Because of their aluminosilicate composition and ability to discriminate small molecules, zeolites are considered as a subclass of molecular sieves. Molecular sieves are crystalline non-aluminosilicate framework materials in which Si and/or Al tetrahedral atoms of a zeolite lattice are substituted by other T atoms such as B, Ga, Ge, Ti, V, Fe, or P.
Zeolite frameworks are usually negatively charged due to the replacement of Si4+ by Al3+. In natural zeolites this charge is compensated by alkali or alkali earth cations such as Na+, K+ or Ca2+. In synthetic zeolites the charge can also be balanced by ammonium cations or protons. Synthetic zeolite and molecular sieves are prepared usually under hydrothermal conditions from aluminosilicate or phosphate gels. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and, often times, in the presence of structure directing agents (templates). The proper selection of template is of extreme importance for the preparation of a particular framework and pore network. A large variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to give more than 85 different molecular sieve framework structures (Meier et al., Atlas of Zeolite Structure Types, Butterworth, London (1992)). An excellent review of the use of various organic templates and their corresponding structures, as well as the mechanism of structure directing is given for example in Gies et al., Zeolites, vol. 12, 42-49 (1992). Due to their uniform pore size, unique crystalline framework structure and ability for isomorphous substitution synthetic zeolites and molecular sieves are extremely suitable for a number of adsorption, separation and catalytic processes involving organic molecules. Recently, it has been discovered that synthetic zeolites and molecular sieves can be functionalized by partially substituting the framework T-atoms with such metal atoms capable of performing different chemical (mostly catalytic) tasks. As a result, a large variety of highly selective catalysts have been reported during the last decade. In the spectrum of molecular sieve catalyst a special place is occupied by the metal-substituted, high silica molecular sieves (Si/Al ratio  greater than 5). Such molecular sieves are highly hydrophobic and therefore exhibit high affinity toward organic molecules. Among these important materials the microporous Ti-substituted high silica molecular sieve, silicalite-1 (denoted TS-1), with MFI structure and pore size of ≈6 xc3x85 is quickly emerging as a valuable industrial catalyst due to its ability to oxidize organic molecules at mild reaction conditions.
The microporous transition metal-substituted zeolites and molecular sieves of the prior art exhibit mainly framework-confined uniform micropores, and little or no textural mesoporosity as evidenced by their Langmuir type adsorption isotherms accompanied with poorly developed hysteresis loops at Pi/Po  greater than 0.4. The typical values for their specific surface area are from 300-500 m2/g and for the total pore volume xe2x89xa60.6 cm3/g (Perspectives in Molecular Sieve Science, Eds. Flank, W. H. and White T. E. Jr., ACS symposium series No. 368, Washington D.C., p. 247; 524; 544 (1988)). All known microporous high silica metallosilicates are prepared by prolonged crystallization at hydrothermal conditions, using single quaternary ammonium cations or protonated primary, secondary or tertiary amines to assemble the anionic inorganic species into a microporous framework. The use in the prior art of neutral amines and alcohols as templates (Gunnawardane et al., Zeolites, vol. 8, 127-131 (1988)) has led to the preparation of microporous highly crystalline (particle size  greater than 2 xcexcm) molecular sieves that lack appreciable textural mesoporosity. For the mesoporous molecular sieves of the MCM-41 family the uniform mesopores are also framework-confined. This has been verified by TEM lattice images of MCM-41 shown in U.S. Pat. No. 5,102,643. Therefore, the framework of this class of materials can be viewed as an expanded version of a hexagonal microporous framework. The existence of these framework-confined uniform mesopores was also confirmed by the capillary condensation phenomenon observed in their adsorption isotherms. The lack of appreciable hysteresis beyond Pi/Po  greater than 0.4 implies the absence of textural mesoporosity. This lack of textural mesoporosity is also supported in some cases by the highly ordered hexagonal prismatic shaped aggregates of size  greater than 2 xcexcm (Beck et al., J. Am. Chem. Soc., vol. 114, 10834-10843 (1992)). The total pore volume of the material reported by Davis et al. is ≈0.7 cm3/g and that of the framework-confined mesopores, as determined from the upper inflection point of that hysteresis loop, is almost equal to that of the total pore volume. Therefore, the ratio of textural to framework-confined mesoporosity here approaches zero. The size of the frameworkxe2x80x94confined uniform mesopores is ≈30 xc3x85.
There is a need for mesoporous molecular sieve structures exhibiting high thermal and hydrothermal stability (i.e., large framework crosslinking and large wall thickness), small particle size and complementary framework-confined and textural mesoporosity. Also, there is a need for a new preparation art to these ordered mesostructures which would allow for cost reduction by employing less expensive reagents and mild reaction conditions while at the same time providing for the effective recovery and recyclability of the neutral template.
Significant progress has been made recently in extending the supramolecular assembly of mesostructured inorganic framework structures to include hierarchical forms with a variety of particle shapes (Ozin, G. A., Acc. Chem. Res. 30 17 (1997); G. A. Ozin, et al., Nature 386 692 (1997); Mann, S., et al., Chem. Mater. 9 2300 (1997); Schacht, S., et al., Science 273 768 (1996); and Lin, H. P., et al., Science 273 765 (1996)). Mesoporous metal oxide molecular sieves with vesicle-like morphologies are of special interest as potential catalysts and sorbents, in part, because the mesostructured shells and intrinsic textural pores of the vesicles should efficiently transport guest species to framework binding sites. However, all of the vesicle-like mesostructures reported to date have shells of undesirable thickness. More importantly, like many mesoporous molecular sieves with conventional particle morphologies, the framework structures defining the vesicle shells are lacking in structural stability. For instance, a vesicular aluminophosphate with mesoscale d-spacing and surface patterns that mimic diatom and radiolarian skeletons collapses to AlPO4-cristobalite with complete loss of the hierarchical patterns at 300xc2x0 C. (Oliver, S., et al., Nature 378 47 (1995)). Also, vesicle-like silicic acid polymers templated by didodecyldimethylammonium bromide lose their hierarchical structures simply upon washing with alcohols (Dubois, M., et al., Langmuir 9 673 (1993)). Macroscopic hollow spheres of mesoporous MCM-41 (Schacht, Q., et al., Science 273 768 (1996); and Huo, Q., et al., Chem. Mater. 9 14 (1997) have been prepared from oil-in-water emulsions, but these particles have shells that are very thick (1000-5000 nm) and comparable in size to mesostructures with conventional particle. The feasibility of forming vesicular mesoporous silicas has been demonstrated recently by an assembly pathway based on the use of an amine bolaamphiphile as the structure directing agent (Tanev, P. T., et al., Science 271 1267 (1996)). However, the shell thickness (100-250 nm) was large compared to the vesicle diameter (300-800 nm). More importantly, the thermal and hydrothermal stability was unremarkable, and the particle shape deviated greatly from the desired vesicular form with increasing surfactant chain length (Tanev, P. T., et al., J. Am. Chem. Soc. 119 8616 (1997)).
Although several surfactant systems are known to direct the assembly of lamellar silica mesostructures (Tanev, P. T., et al., Science 271 1267 (1996); Tanev, P. T., et al., J. Am. Chem. Soc. 119 8616 (1997); Kresge, C. T., et al., Nature 359 710 (1992); J. S. Beck et al., J. Am. Chem. Soc. 114 10834 91992); Ogawa, M., J. Am. Chem. Soc. 116 7941 (1994); and Lu, Y., et al., Nature 389 364 (1997)), none is capable of generating an entire family of thermally and hydrothermally stable lamellar silicas. Related mesoporous silica art using amine templates to assemble mesostructured silicas is described in U.S. Pat. Nos. 5,672,556, 5,712,402, 5,785,946, 5,800,800, 5,840,264 and 5,855,864 to Pinnavaia et al. These products also have relatively poor hydrothermal stability.
The present invention relates to a lamellar mesoporous silica composition containing an electrically neutral gemini amine surfactant in mesopores, the composition having at least one x-ray diffraction peak corresponding to a basal spacing between about 4 and 10 nm and wherein cross-linking of SiO4 tetrahedra to four adjacent silicon sites (Q4) and to three adjacent silicon sites (Q3) corresponds to a Q4/Q3 ratio of at least 5. The notation Q4 and Q3 refers to distinguishable linkages for the SiO4 units in the silica framework. The silicon atoms in Q4 tetrahedra are linked through bridging oxygen atoms to four other silicon centers, whereas the silicon atoms of Q3 tetrahedra are linked to three other silicon centers through bridging oxygens. Q2 and Q1 linkages also are possible, but they occur in very low abundance in comparison to Q3 and Q4 linkages. The siting of the SiO4 tetrahedra in the framework is readily determined by 29Si NMR spectroscopy. Silica frameworks with a high Q4/Q3 ratio are especially desirable. The larger the Q4/Q3 ratio, the more completely crosslinked and, hence, the more stable, is the framework. All previously reported mesostructured silicas exhibit Q4/Q3 ratios near 2.0 in as-synthesized form. Calcining the mesostructures increases the ratio to values near 3.0. The especially large Q4/Q3 values of at least 5.0 are unprecedented for mesostructured silica compositions. Accordingly, these compositions are associated with novel thermal and hydrothermal stability.
The present invention also relates to a lamellar mesoporous metal oxide and silica composition derived from a silica composition containing an electrically neutral gemini amine surfactant in mesopores, which surfactant has been removed from the silica composition which is then treated with a compound containing one or more functional inorganic elements and then heating the treated silica composition to form the metal oxide and silica composition.
The present invention further relates to a lamellar mesoporous silica composition with at least one x-ray diffraction peak corresponding to a basal spacing of between about 4 and 10 nm and wherein cross-linking of the SiO4 tetrahedra of the silica to four adjacent silicon sites (Q4) and to three adjacent silicon sites (Q3) corresponds to a Q4 to Q3 ratio of at least 5.
The present invention also relates to a lamellar mesoporous silica composition with at least one x-ray diffraction peak corresponding to a basal spacing of between about 4 and 10 nm and wherein cross-linking of the SiO4 tetrahedra sites of the silica to four adjacent silicon sites (Q3) corresponds to a Q4 to Q3 ratio of at least 5, and produced from a lamellar silica composition containing a neutral gemini amine surfactant which is removed to produce the lamellar mesoporous silica composition.
The present invention relates to a method for forming a lamellar mesoporous silica composition which comprises: reacting in a reaction mixture a lower alkyl tetraorthosilicate with a gemini amine surfactant to form the lamellar mesoporous silica composition; and separating the composition from the reaction mixture.
Finally, the present invention relates to a lamellar mesoporous silica composition containing an electrically neutral gemini amine surfactant in mesopores defining the silica composition.
The present invention discloses that an electrically neutral hydrogen bonding pathway based on the hydrolysis of a silicon alkoxide, such as tetraethylorthosilicate (TEOS), in the presence of gemini amine surfactants at a temperature in a range between about 50 and 150xc2x0 C. results in the assembly of lamellar silica mesostructures that are mesoporous and exceptionally stable under thermal and hydrothermal conditions. Moreover, the lamellar framework silicas adopt a hierarchical particle structure that is typically vesicular, bowl-like, and ribbon-like in shape. When the assembly is carried out under non-hydrothermal conditions between about 30 and 50xc2x0 C., a wormhole framework structure is formed.
The term xe2x80x9cparticlexe2x80x9d as used herein means a fundamental un-agglomerated object with a hierarchical shape which is determined microscopically. These fundamental particles agglomerate to form larger aggregates of particles. The preferred fundamental particles of the present invention are in large part vesicular or bowl-shaped with a diameter between about 10 and 1400 nm and a shell thickness between about 2.0 and 200 nm which is quite thin and unique in comparison to the prior art. Preferably there are between about 1 and 50 nanolayers of silica providing the thickness of the shell. Each silica nanolayer has an elementary thickness in the range 2.0-10 nm. The shell thickness of a vesicle or bowl is determined by the nesting of the nanolayers, one atop another. The nested nanolayers are separated by lower density silica pillars that create channels and pores between the silica lamellae. Vesicular and bowl-shaped particles provide excellent textural mesoporosity and facilitate access of guest molecules to the adsorption sites and the reaction sites of the lamellar framework structure. Consequently, the compositions of the present invention are especially useful as adsorbents and heterogeneous catalysts.
The nesting and folding of the framework silica nanolayers affords, in addition to vesicles and bowl-like particles, fundamental particles with shapes that may be described as folded ribbons, open-ended tubes, and spheres formed by the concentric nesting of spherically folded silica nanolayers in a manner akin to the assembly of the structural elements of an onium. The shapes of these elegant particle objects all result from the folding and nesting of silica nanolayers with a thickness in the range 3.0-10 nm.
The term xe2x80x9csurfactantxe2x80x9d means a surface active agent wherein the molecule has a hydrophobic segment adjacent to a hydrophilic polar head group. The term xe2x80x9cgemini amine surfactantxe2x80x9d means that the hydrophilic polar head group of the surfactant contains at least two (2) amino groups separated by 1 to 4 carbon atoms. There also can be more than two of the amino groups contributing to the polarity of the hydrophilic head group. The gemini amine surfactants that are especially preferred in forming the lamellar mesostructures of the present invention have the following general molecular structure represented by the formula:
RNH(CH2)mNHR1
wherein R is a hydrophobic segment, R1 is hydrogen, methyl, ethyl or a xe2x80x94(CH2)mNH2 group and m is an integer between 1 and 4. An example of a suitable hydrophobic segment is an aliphatic group of the type CnH2n+1 where n=8 to 20. Gemini amine surfactants of the type
CnH2n+1NH(CH2)2NH2
are especially effective in forming the mesostructured compositions of the present invention. Linsker, R., et al., J. Am. Chem. Soc. 67 1581 (1945) describes the synthesis of some of these gemini amine surfactants. Some are available commercially.
The pure mesostructured silica compositions of the present invention are useful as adsorbents and supports for catalysts of different kinds. For instance certain acids, such as phosphoric acid and heteropolyacids of the type H3PW12O40, H4SiW12O40 and polyoxometallate related species, can be supported in the framework pores of the pure lamellar silica mesostructures. Highly dispersed metals in reduced elemental form, such as nickel, platinum, palladium, iridium, rhodium, rhenium, ruthenium, as well as metal oxides, such as SnO2, V2O5, ZrO2 and many other oxides, and sulfides, such as Co/Mo sulfides, can be supported as nanosized particles in the framework pores of the pure silica mesostructures. All of the said catalytic compounds can be introduced into the framework pores of the lamellar mesostructure through the impregnation of suitable precursors by methods well known to those skilled in the art of catalyst formulations.
Although the pure lamellar silica mesostructures of the invention are useful as high surface area adsorbents and catalyst supports, they become even more useful as catalysts and adsorbents when they are functionalized through the introduction of one or more reactive inorganic elements into the lamellar framework structure of the mesostructured silica. The introduction of one or more functional organic groups into the silica framework, such as those with sequestering properties for metal ions, organic acid-base properties, or any of a large variety of organic functional groups for chemical catalysis and for cross-linking the silica framework to a polymer matrix, also greatly extends the utility of the lamellar mesoporous silicate compositions. Organic functional groups with metal ion sequestering properties, when integrated into the framework of the lamellar silica, behave as metal ion traps for the removal of toxic metals from contaminated water and waste streams. Organofunctional groups with acidic or basic properties allow the functionalized silicas to be used as acidic or alkaline catalysts. Other organic functionalities, such as acid anhydride, epoxide, vinyl and many other well-recognized reactive moieties, allow the framework to be linked through chemical bond formation to other organic guest molecules in the framework pores and at the external surfaces of the lamellar silica mesostructures. These latter hybrid compositions have many desired properties.
A lamellar mesoporous composition wherein the composition has the formula MxSi1xe2x88x92xOq when written in anhydrous form where M is an inorganic element other than silicon or oxygen, x is between about 0.001 and 0.35, and y is between about 1.80 and 2.25, the composition having at least one x-ray diffraction peak corresponding to a basal spacing between about 4 and 10 nm and wherein cross-linking of SiO4 tetrahedra to four adjacent silicon sites (Q4) and to three adjacent silicon sites (Q3) corresponds to a Q4/Q3 ratio of at least 5.
One or more other inorganic elements can be incorporated into the silica framework structures of this invention to form lamellar mixed oxides. These functionalized mixed oxide derivatives have compositions of MaObxe2x80x94SiO2 when written in anhydrous form, wherein M is a least one element other than silicon or oxygen and a and b provide an elemental balance in MaOb. Alternatively, the compositions of the functionalized lamellar mixed oxide derivatives of the present invention expressed by the following formula when written in anhydrous form:
MxSi1xe2x88x92xOq
where M is at least one element which forms an oxide and is preferably selected from the group consisting of B, Al, Ga, Fe, Co, Mn, Cr, Ge, Ti, V, Ni, Sn, Sb, Zr, W, Mo, Ca, Cu, Pb, In, Nb, Sr, and Zn. In the calcined composition x is between 0.001 and 0.35 and q is between about 1.5 and 2.5. Preferably x is between about 0.005 and 0.35. Preferably q is between about 1.80 and 2.25.
Two general methods may be used to introduce one or more reactive inorganic elements into the said mixed oxide compositions of the lamellar silica mesostructures of this invention. One method is to incorporate the desired element or elements into the framework as the framework is being assembled in the presence of the gemini amine surfactant. This is the so-called xe2x80x9cdirect assemblyxe2x80x9d pathway to the said MxSi1xe2x88x92xOq oxide compositions. Alkoxides of the desired element or elements are preferred as precursors, in part, because they are miscible with the silicon alkoxides that are used as precursors to the silica framework. The second, more preferred, method is the so-called xe2x80x9cpost-synthesis reactionxe2x80x9d method wherein the desired element or elements are introduced into the silica framework through the reaction of the pre-assembled lamellar silica framework structure with a desired element precursor. The post-synthesis reaction may be carried out in the presence of the gemini surfactant in the framework mesopores or, more preferably, the surfactant can be removed from the silica framework mesopores through solvent extraction or calcination prior to reaction of the framework with the desired element precursors to afford the desired MxSi1xe2x88x92xOq compositions.
Ordinarily, the said direct synthesis method provides lamellar MxSi1xe2x88x92xOq compositions wherein the value of x is low, typically in the range x=0.001-0.01. At higher values of x, and depending on the element or collection of elements represented by M, the lamellar framework structure is lost. Alkoxides are the most desired precursors for introducing functional M elements into the silica framework by the direct synthesis route. Alkoxides of many di-, tri-, tetra-, penta- and hexavalent elements are known and many of these are commercially available. One or more of the precursor alkoxides may be mixed with the precursor alkoxide of silicon in producing the desired MxSi1xe2x88x92xOq functional compositions.
The said post-synthesis route to the lamellar MxSi1xe2x88x92xOq compositions of this invention is preferred. Because the lamellar framework is pre-assembled, the lamellar framework, as well as the hierarchical particle morphology, is retained upon insertion of the reactive M centers into the framework. Typically, one or more M species may be inserted into the lamellar silica framework at x values over the range 0.001-0.35. Alkoxides are suitable reagents for the post-synthesis pathway to the desired lamellar MxSi1xe2x88x92xOq compositions. However, post-synthesis pathway is not limited to the use of alkoxides as reagents for the insertion of M centers into the lamellar silica framework. Many lower-cost salts and metal complexes also are suitable reagents for the post-synthesis pathway to lamellar MxSi1xe2x88x92xOq compositions. For instance, aluminum centers can be introduced into the preassembled lamellar silica framework by reaction of the as-synthesized, solvent-extracted, or calcined forms of the silica mesostructure with salts of aluminum, such as aluminum nitrate and sodium aluminate. Any aluminum salt is effective in inserting aluminum centers into the silica framework. MxSi1xe2x88x92xOq compositions with M=Al are especially desired because the aluminum centers introduce acidic sites that are useful in catalyzing important organic chemical conversions, such as alkylation reactions and cracking reactions.
The present invention provides a route to the synthesis of organic-inorganic metal oxide compositions with well defined framework-confined mesopores. The compositions produced in the current invention are distinguished from those of the prior art by the virtue of the method of preparation of the present invention, the subsequent architecture of the mesoporous structure and the range of templated organic-substituted metal oxides that is afforded by this route. Formation of the mesoporous network is accomplished by interaction (complexation and/or hydrogen-bonding) between a gemini amine surfactant template and neutral inorganic and organic precursors, followed by hydrolysis and subsequent condensation of the inorganic reaction product under either ambient or elevated temperature reaction conditions and the subsequent removal of the solvent phase and the template. The compositions also have intra- and interparticle textural mesoporosity, in addition to framework mesoporosity.
The present invention particularly provides a preferred nonionic route to the preparation of quasi-crystalline inorganic-organic oxide compositions comprising (a) preparing a homogeneous solution or emulsion of a gemini amine surfactant by stirring, sonicating or shaking at ambient temperature and pressure; (b) addition of one or more of each of neutral inorganic and organic precursors with stirring at ambient temperatures and pressures to the emulsion of step (a) at ambient temperature to form a precipitated semi-crystalline product; allowing the mixture to age for a period of time at a temperature between 85xc2x0 and 150xc2x0 C., more preferably between 100xc2x0-120xc2x0 C.; (c) separating the solvent and the hydrolyzing agent from the precipitated product by filtration or centrifugation; and (d) extracting the template through solvent extraction whereby the solvent is either water or ethanol, or an ethanol-water mixture, or alternatively removing the surfactant through calcination at temperatures in excess of 400xc2x0 C.
The present invention thus provides a new route to mesostructured silica compositions with uniform, well defined, framework-confined mesopores that can be utilized as adsorbents, metal ion traps, solid acids and bases, and catalysts and catalyst supports for the catalytic conversion of organic substrates. According to the method of the present invention the formation of the mesoporous structure is accomplished primarily by interaction (complexation and/or hydrogen bonding) between template molecules within micellar aggregates of a gemini amine surfactant and neutral organic and inorganic oxide precursors, followed by hydrolysis and cross-linking of IOx units, wherein between 65 and 100% of the I units are SiO4 units and the remainder are metallic or non-metallic elements coordinated to x oxygen atoms (2xe2x89xa6xxe2x89xa68). This interaction is most likely to occur between an Ixe2x80x94OH unit and the NH2 functions of each surfactant molecule, or between the Ixe2x80x94OH unit and the array of lone pair electrons on the template polar segment. The polar segment of the template in the present invention is flexible and appears to act in the fashion of a ligand complexing through hydrogen bonding to a Ixe2x80x94OH unit, thereby stabilizing a site of nucleation for subsequent condensation of the mesoporous quasi-crystalline organic and inorganic oxide product, although the inventors do not want to be bound to any particular theory.
The prior art does not describe the preparation of micro-, meso-, or macro-porous inorganic oxide compositions by such a nonionic mechanism involving crystallization of organic and inorganic oxide precursors around well defined micelles of the gemini amine surfactants. Specifically, the present results are achieved by using micelles of the surfactant to template and assemble neutral inorganic and organic reactant precursors into a mesoporous framework structure. Complexation and/or hydrogen bonding between the template and the reagent is believed to be the primary driving force of the assembly of the framework in the current invention. The aforementioned method consists of the formation of a solid precipitate by the mixing of a solution or emulsion of electrically neutral gemini amine surfactant, with a neutral inorganic, usually inorganic alkoxide, and an organic oxide precursor in the presence of a hydrolyzing agent, followed by aging assembly temperatures between ambient and 150xc2x0 C. for at least 8 hours. The template may be recovered by extraction with ambient temperature alcohol or hot water-ethanol mixtures or it can be removed by calcination at temperatures above 500xc2x0 C. Thus both the compositions and methodologies of the present invention differ fundamentally from previous art in which cationic gemini surfactants are used to assemble mesostructures (Huo, et al., Science, 268 1324 (1995)). When the gemini surfactant is made cationic through the presence of quaternary ammonium ions on the polar head group, as disclosed in the prior art of Huo et al, the assembly reaction is under electrostatic (columbic) control and different mesostructured frameworks are formed.
The template may be removed from the condensed reaction products by solvent extraction of the template from the air dried material using an organic solvent such as an alcohol or using hot water or using a hot alcohol-water mixture.
The synthesis methods for the formation of MSU-G materials involve the preparation of solutions or emulsions of a gemini amine surfactant template compound and reaction of this solution with a silicon alkoxide precursor and optionally including di-, tri-, tetra-, penta- or hexa-valent metal or metalloid hydrolyzable alkoxide reagents and optionally including an organic silane in the presence of a hydrolyzing agent under static, stirring, sonication or shaking and conditions at temperatures in the range 85-150xc2x0 C. until formation of the desired precipitated product is achieved and recovering the solid material.
The inorganic oxide precursors are single or double metal alkoxide compounds, The list of preferred alkoxides includes but not exclusively: aluminum(III) ethoxide, aluminum(III) isopropoxide, aluminum(III) n-, sec- or tert-butoxide, antimony(III) isopropoxide, antimony(III) n-butoxide, calcium(II) ethoxide, calcium(II) isopropoxide, calcium(II) tert-butoxide, chromium(IV) isopropoxide, chromium(IV) tert-butoxide, copper(II) methoxyethoxide, gallium(III) isopropoxide, germanium(IV) ethoxide, germanium(IV) isopropoxide, indium(III) isopropoxide, iron(III) ethoxide, iron(III) isopropoxide, iron(III) tert-butoxide, lead(II) isopropoxide, lead(II) tert-butoxide, magnesium(II) ethoxide, manganese (II) isopropoxide, molybdenum(V) isopropoxide, niobium(V) ethoxide, silicon (IV) methoxide, silicon(IV) ethoxide, silicon(IV) propoxide, silicon(IV) butoxide, silicon(IV) hexoxide, strontium(II) ethoxide, tin(IV) isopropoxide, titanium(IV) ethoxide, titanium(IV) propoxide, titanium(IV) isopropoxide, titanium(IV) butoxide, titanium(IV) octadecoxide, tungsten(VI) ethoxide, tungsten (VI) isopropoxide, vanadium(V) triisopropoxide oxide, zinc(II) isopropoxide, zinc(II) tert-butoxide, zirconium(IV) n-propoxide, zirconium(IV) isopropoxide, zirconium(IV) butoxide, zirconium(IV) tert-butoxide, aluminum(III) silicon(IV) alkoxide, titanium(IV) silicon(IV) polyethoxide and other mixtures of the aforementioned alkoxide compounds. The alcohols used in step (i) of the preparation art correspond to the alcoholate ligand from which the metal alkoxide is derived. The alcohols thus preferred are methanol, ethanol, n- and isopropanol and n-, sec-, tert-, butanol. The alcohols contain 1 to 4 carbon atoms.
A lamellar mesoporous composition wherein the composition has the formulas selected from the group consisting of [(R1)SiO3/2]xSiO2, [(R1)2SiO]xSiO2, and [(R1)3SiO1/2]xSiO2 and mixtures thereof, when written in anhydrous form where R1 is an organic moiety containing an organic functional group and x is between about 0.01 and 0.30, the compositions having at least one x-ray diffraction peak corresponding to a basal spacing between about 4 and 10 nm and wherein cross-linking of SiO4 tetrahedra to four adjacent silicon sites (Q4) and to three adjacent silicon sites (Q3) corresponds to a Q4/Q3 ratio of at least 5.
The silica composition of this invention also can contain a substituted organo silane as discussed, for example, by Richer et al., Chem. Commun. 1775-1776 (1998) and Brown et al., Chem. Commun. 69-70 (1999). The materials provide unique surface properties in the as-formed silica composition. These compositions can be produced by direct incorporation of an organosilane during the assembly of the lamellar silica framework or the organo groups can be introduced by post-synthesis reaction of the surface hydroxyl groups of the framework with a suitable organosilane. The post-synthesis method is preferred.
These organo functional derivatives of the lamellar mesostructures of the present invention have the following compositions when written in anhydrous form: [(R1)SiO3/2]xSiO2, [(R1)2SiO]xSiO2 and [(R1)3SiO1/2]xSiO2 where R1 is an organic moiety, preferably containing an organo functional group and X is between about 0.01 and 0.30. These compositions have about the same range of XRD basal spacings and XRD and Q4/Q3 ratios as have been described above for the pure silica frameworks.
The organic silanes which are useful in producing the said organo-functional lamellar silica compositions of the present invention are those which will react to form the mesoporous structure in a direct-synthesis pathway and those that will react with the pre-assembled lamellar silica in a post-synthesis reaction pathway. Included are preferred silanes of the formula:
(RO)3SiR1, (RO)2Si(R1)2 and ROSi(R1)3,
where R is hydrogen or a lower alkyl group (1 to 8 carbon atoms). The general formula is Si(OR)4xe2x88x92n(R1)n where n is 1, 2 or 3. Another way of expressing the useful silane reagents for the introduction of organic functionality into the lamellar silica framework is through the general formulas X3Si(R1), X2Si(R1)2 and XSi(R1)3 where X is any hydrolyzable group which reacts with silica. R1 in the above formulas can be the same as R and can be an organic group which is non-reactive which can be an alkyl, alkoxy, alkenyl or alkynyl, cycloaliphatic, aromatic group containing 1 to 25 carbon atoms. R1 can include substituents of O, N or S and can provide a hydroxide, an aldehyde, acid, base, sulfide, cyanide, mercaptan and the like. Examples of basic moieties especially useful as catalysts include amines and pyridyl groups. Useful acidic functionalities include carboxylic acids, sulfonic acids, and fluorinated sulfonic acids. R1 can contain a halogen selected from the group consisting of F, I, Br or Cl and the R1 group can be further reacted at the halogen group. The preferred R1 contain moieties which are metal binding to provide selective adsorption of metal ions from solution. Especially useful metal trapping agents include organic groups containing chelating ligands such as ethylene diamines, ethylene diamine tri- and tetra acetate, cyclic and bicyclic polyethers known as crown ethers and cryptans and the like. The mixed metal alkoxides and organic-alkoxy silanes can be obtained commercially. Alternatively, they may be specifically prepared for use in forming the desired compositions. Functional organosilanes can be prepared by hydrosilylation of olefins. The desired organo-functional lamellar silica compositions may be prepared most preferably by reaction of the parent lamellar silica with a said organosilane under reflux in toluene for 3-4 hours.
Preferably the R1 group of the said Si(OR)4xe2x88x92n(R1)n silanes contains a functional group selected from a metal complex, vinyl, cyano, amino, mercapto, halogen (usually Cl or Br), aldehyde, ketone acid (including sulfonic and F-sulfonic acid or base group). The metal complex functionality helps form structures where the metal is removable and provides increased receptivity to the metal removed.
Examples of commercially available functional silanes which can be used are:
3-(N-allylamino)propyltrimethoxy-silane;
O-allyloxy(polyethyleneoxy)-trimethylsilane;
N-(2-aminoethyl)-3-aminopropylmethyl-dimethoxysilane;
N-(2-aminoethyl)-3-aminopropyltri-methoxysilane N-[3-(trimethoxysilyl)propyl]ethylenediamine;
N-(6-aminohexyl)aminopropyl-trimethoxysilane;
2-[Methoxy(polyethyleneoxy)propyl]trimethoxysilane;
(3-Trimethoxysilylpropyl)diethylene-triamine 95%;
Trivinylmethoxysilane;
3-Cyanopropyldimethylmethoxysilane;
3-Cyanopropyltriethoxysilane;
(3-Cyclopentadienylpropyl)triethoxysilane;
Diphenyldiethoxysilane;
Diphenyldimethoxysilane;
Diphenylsilanediol;
Diphenylvinylethoxysilane;
(Mercaptomethyl)dimethylethoxysilane;
(Mercaptomethyl)methyldiethoxysilane;
3-Mercaptopropylmethyldimethoxysilane;
3-Mercaptopropyltrimethoxysilane;
3-Mercaptopropyltriethoxysilane;
3-Methacryloxypropyldimethylethoxy-silane;
3-Methacryloxypropyldimethylmethoxysilane;
3-Methacryloxypropylmethyldiethoxy-silane;
3-Methacryloxypropylmethyldimethoxysilane;
3-Methacryloxypropyltrimethoxysilane;
Methylphenyldimethoxysilane;
Methyl [2-(3-trimethoxysilylpropylamino)-ethylamino]-3-propionate (65% in methanol);
7-Oct-1-enyltrimethoxysilane;
Phenethyltrimethoxysilane;
N-Phenylaminopropyltrimethoxysilane;
Phenyldimethylethoxysilane;
Phenyltriethoxysilane;
Phenyltrimethoxysilane;
Phenylvinyldiethoxysilane;
N-[3-(triethoxysilyl)propyl]-4,5-dihydro-imidazole;
2-(Trimethoxysilyl)ethyl-2-Pyridine;
Trimethoxysilylpropyldiethylenetriamine (95%);
N[(3-trimethoxysilyl)propyl]ethylene-diamine triacetic acid, trisodium salt (50% in water);
N-(3-trimethoxysilylpropyl)pyrrole;
Triphenylsilanol;
Vinyldimethylethoxysilane;
Vinylmethyldiethoxysilane;
Vinyltriethoxysilane;
Vinyltrimethoxysilane;
N-(trimethoxysilylpropyl)ethylene-diamine, triacetic acid, trisodium salt;
4-Aminobutyldimethylmethoxysilane;
4-Aminobutyltriethoxysilane (95%);
N-(2-aminoethyl)-3-aminopropylmethyldi-methoxysilane;
H2NCH2CH2CH2SiOEt 3-aminopropyldimethylethoxysilane;
3-Aminopropylmethyldiethoxysilane;
3-Aminopropyldiisopropylethoxysilane;
3-Aminopropyltriethoxysilane;
3-Aminopropyltrimethoxysilane;
N-(triethoxysilylpropyl)urea (50% in methanol).
N-[3-(triethoxysilyl)propyl]phthalamic acid (95%).
The above list is not meant to be limiting, but merely intended to provide examples of easily obtainable functional silanes useful in preparing the compositions of this invention. In general, any functional organosilane may be incorporated into the porous inorganic and organic oxide materials embraced by the present art. Functional organo groups on the silane moiety that are especially useful include acids and bases for catalytic applications, dye chromophores for sensing, linking groups (e.g. epoxides, acid anhydrides, amines, esters, carboxylates and the like) for forming polymer-inorganic nanocomposites, and complexants for binding metal ions. In the latter case the bound metal ions may be recovered by ion exchange or subsequently used in the immobilized state as catalysts for organic chemical transformation.
The templated mesostructured silica compositions of the present invention can be combined with other components, for example, zeolites, clays, inorganic oxides or organic polymers or mixtures thereof. In this way adsorbents, ion-exchangers, catalysts, catalyst supports or composite materials with a wide variety of properties may be prepared. Additionally, one skilled in the art may impregnate or encapsulate transition metal macrocyclic molecules such as porphyrins or phthalocyanins containing a wide variety of catalytically active metal centers.
Additionally, the surfaces of the compositions can be functionalized in order to produce catalytic, hydrophilic or hydrophobic surfaces. The surfaces may be functionalized after synthesis by reaction with various chlorides, fluorides, silylating or alkylating reagents.
In the present invention the gemini amine surfactants can be removed from the as-synthesized mesostructures by solvent extraction to form surfactant-free mesostructures. Useful solvents include polar molecules such as water, alcohols, ketones, nitrides and the like. Preferred is ethanol. The surfactant can also be removed from the as-synthesized mesostructure by calcination. However, preferably the surfactant is removed prior to calcination and can be recycled.
Calcination of the as-synthesized and solvent-extracted mesostructures is achieved at temperatures between about 200xc2x0 and 1000xc2x0 C. Furnaces for accomplishing calcination are well known to those skilled in the art.